1. Field of the Invention
The present invention relates to a vehicular air bag system, and more particularly to a diagnostic apparatus for testing a squib line system in such an air bag system.
2. Description of the Related Art
An air bag system is employed to protect passengers in the event of a vehicular collision. Such an air bag system includes, for example, a sensor installed in a vehicle to detect an impact, and an air bag installed in the steering wheel to protect the driver against the impact of a collision. If the sensor detects impact on the vehicle, a large current flows to the squib installed in the air bag, and the generated heat ignites the initiating explosive located close to the squib. Then, due to a chemical reaction initiated by the heat of the explosion, a large volume of nitrogen gas is generated in the air bag in a fraction of a second and inflates the air bag. When the air bag system is in use, the system must be monitored at all times for safety and proper operation.
There have previously been proposed squib line system diagnosis apparatus designed for vehicular air bag systems. One such apparatus is disclosed in Japanese Patent Unexamined Publication No. 1-306343. However, with the previously proposed apparatus, it is not possible to identify the source of a failure with sufficient accuracy. More specifically, it is not possible, for example, to distinguish between a failure of a resistor installed for diagnostic purposes and a failure of an internal resistor installed in the impact detection sensor. Furthermore, when a squib terminal is short-circuited to a battery line, there is a possibility that the trouble may be mistaken for a failure in a different part if the value of the voltage on the battery line is low.